For many years, fluorescent lamps were the most common type of light fixture used in commercial lighting applications. These traditional lighting fixtures use fluorescent bulbs in recessed troffers with parabolic reflectors. The most common fluorescent bulbs are the linear T5 (⅝ inch diameter), T8 (1 inch diameter), and T12 (1½ inch diameter) bulbs. However, light emitting diodes (LEDs) can more efficiently convert electrical energy into light than can fluorescent bulbs. Thus, efforts have been made to incorporate LEDs into tubes having the form factor of T5, T8 and T12 fluorescent bulbs. LEDs can then be used to retrofit traditional fluorescent light fixtures by installing LED bulbs in the troffers that were originally designed for fluorescent bulbs. The existing fluorescent lamp ballasts must then be replaced with new power supply drivers for the LEDs.
The LED bulbs are typically configured as strings of series-connected LEDs coupled in parallel. Multiple LEDs are connected in series to form individual LED strings, and then the LED strings are coupled in parallel to a common voltage and current source. The buck converter of the LED driver can operate at a higher efficiency if the wall voltage powering the LED bulb is a multiple of the driver's output voltage. Thus, a popular LED driver in North America converts a 120-volt wall voltage into an output voltage of thirty or forty volts. The band gap of gallium-nitride LEDs commonly used for lighting is about 3.3 volts. So to correspond to the output voltage of a popular LED drive, the voltage drop across a string of series-connected LEDs can be set at about thirty volts by connecting nine LEDs in the string. A buck converter can then efficiently convert the 120-volt wall voltage to the thirty volts required to light the string of nine LEDs.
FIG. 1 (prior art) shows an LED bulb 10 formed by mounting seventy-two packaged LEDs onto a long substrate anchored in a tube 11 that resembles a T8 bulb. The transparent curved cover is not shown in FIG. 1. The LEDs have been connected as eight series-connected strings of nine LEDs each. The eight strings are connected in parallel to power and ground. An LED driver housed below the long substrate converts the 120-volt wall voltage to the thirty volts required to light each of the eight strings of nine LEDs. In the LED bulb 10 shown in FIG. 1, an entire series-connected string of LEDs is not lit. The third string from the left is not lit because one of the LEDs in the string is defective or because a solder connection in the string shorted out. If the circuit is broken at any location along a series-connected string, the entire string goes dark. As LED bulbs are made with larger numbers of LEDs, the probability that a soldering or LED defect will occur rises much faster than merely the proportional increase in the number of LEDs.
Consumers tend to notice a dark portion of an LED bulb caused by a shorted string of LEDs. Even if the amount of light generated by the LED bulb with the dark portion is nearly as bright as the light produced by a bulb with all of the LEDs lit, the consumer will still perceive the dark portion as a defect and is likely to return the LED bulb. Thus, manufacturers of LED bulbs are faced with higher percentages of returned LED bulbs as bulbs with larger numbers of LEDs are being sold.
A method is sought for packaging LED dies in tubes resembling fluorescent bulbs so that inevitable LED and electrical connection defects will not result in a return rate that rises faster than the increase in the number of LEDs in the LED bulbs.